Environmental Engineering Reference
In-Depth Information
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FIGURE 13.11 The relationship between epilimnetic silicon and biomass of the diatom, As-
terionella, in Lake Windermere, England. Note how decreases in silicon correspond with high
densities of diatoms (data from Lund, 1964).
entire protein). However, iron is required as a minor nutrient and also
serves as a key link among many biogeochemical interactions, such as the
phosphorus and sulfur cycles.
Iron occurs as a dissolved ion in several forms. Under high redox (oxic
conditions), iron takes the form of ferric (Fe 3 ) ions. At low redox iron is
in the ferrous (Fe 2 ) form. O 2 converts ferrous to ferric iron, but the
process occurs over many minutes (Stumm and Morgan, 1981). Ferric ions
react with hydroxyl ions to form the flocculent precipitate ferric hydrox-
ide [Fe(OH) 3 ] under oxic conditions and near-neutral pH. Those who ob-
tain water from anoxic wells with iron-rich groundwater can directly ob-
serve this precipitation. The water appears clear when it is withdrawn, but
exposure to O 2 converts the ferrous iron to ferric iron, which then forms
a flocculent precipitate as ferric hydroxide; a clear glass of water looks a
bit rusty after a few minutes.
Inorganic iron concentrations vary seasonally in stratified lakes, with
especially pronounced variation in concentrations related to anoxic events
such as summer stratification in eutrophic lakes (see Fig. 11.7). These pat-
terns are tied closely to the redox of the lake.
Higher concentrations of iron can be maintained in oxic solutions by
the activity of chelators . These compounds briefly bind iron and other met-
als and do not allow precipitation. Most chelators in natural waters are or-
ganic compounds, although inorganic compounds can also serve as chela-
tors. Some organic chelators of iron are produced by algae and excreted
into solution to keep iron available for uptake. Other substances, such as
humic materials formed by plant decomposition, are also chelators.
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